It is customary to employ tags or pellets which are embedded in an object for the purpose of subsequent identification. For instance, in the case of animals, tags are attached to the skin or subcutaneously and contain some form of a passive circuit which can be inductively coupled or electromagnetically sensed and read into a computer along with other data concerning the condition of the animal.
In protecting articles against theft, similar approaches have been devised to those of animal identification where, for instance, an article can be recognized or identified by matching of a coded object containing a passive circuit with some form of remote sensor or detector. For example, U.S. Pat. No. 3,752,960 to Walton is representative of this approach. Another approach is typified by U.S. Pat. No. 4,309,904 to W. R. Jones et al in which pellets of different thicknesses are embedded in the article and can be ultrasonically sensed. This requires utilization of pellets of different known thicknesses where the thicknesses are selected to represent a specific code or means of identification for that article. U.S. Pat. No. 4,208,795 to H. R. Muhlemann et al discloses implantation of an, information carrier into a cavity and the covering of that carrier and cavity with a visually distinguishable filler. U.S. Pat. No. 4,239,261 to R. H. Richardson is directed more to application of a transparent seal over a micro-marking label which is applied to an article for identification. There the label is placed upon the surface of the article to be identified and secured thereto by the sealant but is visible to the naked eye on close inspection and can be readily removed. U.S. Pat. No. 4,168,586 to P. L. Samis is similar to Muhlemann et al in that it discloses a marker embedded in a cavity and which can be located by means of radiography. However, it is extremely desirable in identification systems to employ a series of markers or portions in the form of microdots which can be rapidly but securely placed at one or more locations on the surface of an article to be identified and in such a way as to be substantially invisible to the naked eye but identifiable by one knowing the locations of the markers. In this way, it is possible to obviate the use of remote sensors or detectors because of the inherent danger that the one stealing the equipment could use such remote means of detection to ascertain the location of the coded marker and remove same along with any other means of identification on the article or equipment. Moreover, it is desirable to provide a method and means of identification which are relatively inexpensive and easy to install thereby facilitating installation at a number of preassigned locations and other random locations if desired and can be completely concealed from view.